A conventional power source for wire cut electrical discharge machining is diagrammatically shown in FIG. 1. An electrode wire 10 is unwound from a reel not shown, fed through an initial hole 100 in the material 12 to be machined, and wound on a reel not shown. A condenser 14 is connected between the electrode wire 10 and the material 12 in parallel thereto. A currentlimiting resistance 18 and a switching transistor 20 are connected in series to a circuit connecting the condenser 14 and a DC power source 16. An oscillator 22 generates an ON-OFF signal which turns on and off the switching transistor 20 to apply a pulsating voltage (and current) between the poles. According to the power source device shown in FIG. 1, the condenser 14 is charged through the resistance 18, and if the insulation between the poles is broken to cause a discharge, the energy stored in the condenser 14 is discharged between the poles, while the charge remains stored in the condenser 14 if no discharge takes place. The maximum average current (Imax) supplied between the poles depends on the electrical conditions, such as the duty factor D and resistance R of a pulse from the switching transistor 20, and is expressed by the following equation: ##EQU1## in which E stands for the voltage of the power source 16.
In electrical discharge machining, no discharge takes place immediately upon application of a voltage between the poles, but there usually occurs a time lag which is called no-load time. The average current I is, therefore, low during actual machining. The average current I is generally proportional to the speed at which the material 12 to be machined is fed, and increases with an increase in the speed if the electrical conditions are not changed.
Thus, the maximum average current Imax is the maximum value of the current which can be supplied for the circuit of a power source for wire cut electrical discharge machining, and the average current value based on the assumption that there is not any no-load time. The average current I is the average current obtained when there is some no-load time, i.e., during actual machining operation, and varies with the progress of the operation. The current I has hitherto been about 8 A at maximum.
As the speed at which the material to be machined is fed is increased, the average current I increases, and if it exceeds about a half of Imax, the machining operation becomes very unstable. In order to increase the material feeding speed, therefore, it is not sufficient to increase I, but it is also necessary to increase Imax. There is, however, a limit to the current which can be supplied to the electrode wire 10, and if a current I.sub.0 exceeding the limit is supplied thereto, the wire 10 is broken. The threshold current I.sub.0 depends on, for example, the material and diameter of the electrode wire 10. The stability of the machining operation is obtained if the average current I is lower than the threshold current I.sub.0. In other words, there is no wire breakage if I and Imax are lower than I.sub.0. If Imax is lower than I.sub.0, there is no fear of the wire being broken, even if I may become very close to Imax on rare occasions due to an external disorder, or changes in the operating conditions, such as non-uniformity in the thickness of the material to be machined, or during the instability of operation which may occur during the machining of a corner. If I exceeds about a half of Imax, however, the operation lacks stability; therefore, it is usually possible to supply only a current which is lower than a half of the threshold current I.sub.0, resulting in a reduction of the material feeding speed to about a half of the ideal speed. The ideal speed is the speed at which the value of I is equal to that of I.sub.0.
According to the conventional power source disclosed in Japanese Patent Publication No. 13195/1969, no condenser is connected between the poles in parallel thereto, but an electric current is supplied between the poles directly by the ON-OFF operation of a switching transistor. According to this system, an electric current is supplied between the poles for a predetermined length of time if the appearance of a discharge is detected by application of a voltage between the poles, and then, the supply of the current is discontinued for a predetermined length of time. This system makes it possible to control uniformly the peak value of the discharge current supplied between the poles after the appearance of a discharge has been detected, and the duration of its supply, and thereby increase the machining speed in accordance with the surface roughness of the material to be machined. This system, however, has the disadvantages hereinabove pointed out of the conventional power source shown in FIG. 1. If I exceeds about a half of Imax, the operation becomes unstable; therefore, it is possible to employ only a current which is lower than a half of the threshold current I.sub.0 and achieve a material feeding speed which is only lower than about a half of the ideal speed.